You can download the data sheets here and compare:http://www.hasselbladhistorical.eu/HW/HWLds.aspx
The one labeled "IF" is internal focusing, and the double focus ring present in the other version is now gone. The "IF" version is essentially a completely new lens, the highest performing 40mm medium format lens on the market, but at a price. The lowest price for a used on on eBay I saw was in the $4000USD range. There are a few new ones left here and there, but they go for well over $5000.